Source of Motions upon the Earth. 153 



Consideration, I think, will shew, however, that almost all of these 

 motions and forces are secondary to the others, and depend, not only 

 in a general way, upon circumstances brought about by their agency, 

 but that, in many cases, they are the direct effects, or accompani- 

 ments, or modifications, of the motions already considered. Thus, 

 for example, the chief terrestrial source of heat or fire, and of light, 

 is from organic matter, — from coal, wood, charcoal, naphtha, oils, 

 and the like. All of these are of organic origin, and are, therefore, 

 the products, as we have seen, of the forces of life and of sunlight. 

 Heat and light are the effects of the passage or transformation of this 

 organic matter, or fuel, from an organic to a neutral and inorganic 

 state. At the ordinary temperatures, and by the unaided chemical 

 forces, this takes place so slowly, and is distributed through so long 

 a time, that the heat and light are not in general perceptible. But 

 if the chemical affinities be intensified, or rendered more active by 

 the aid of other heat, or, m other words, if the fuel be kindled, we 

 have the change effected with rapidity, and the forces which it might 

 otherwise have taken centuries to evolve, are concentrated into a very 

 short time.* Fuel may, therefore, be regarded as a reservoir of 

 power, or of force with a tendency to action, which is the equivalent 

 of the vital and solar forces employed in its production ; or, in other 

 words, the force which we have concentrated in fuel, is equal in 

 amount to the solar and vital forces which formed it. It resembles, 

 therefore, the force by which an arrow is propelled, — a force which, 

 although immediately proceeding from the elasticity of the bow, is 

 nevertheless called into existence by, and may be said to be trans- 

 mitted from, the hand which draws it ; and, in like manner, we may 

 say that the motions which we produce by fire are the eftects of lii'e 

 and sunlight. 



With regard to what is called the nervous force of animals, or 

 that which gives rise to the voluntary and involuntary motions af- 

 fecting the whole or particular parts of animal bodies, we shall only 

 remark, that it is secondary to, and dependent for its existence upon, 

 life and sunlight, inasmuch as it operates and manifests itself in no 

 other way than through a fitly organised body, which is the product 

 of those forces. Many different views of the nature of this force have 

 been held by physiologists. With this subject, however, we have 

 nothing to do at present ; but we may cursorily observe, that it might 



* It may possibly appear that this statement of the nature of combustion is 

 not very accurate, inasmuch as no mention is made of the chemical combina- 

 tion of the carbon and hydrogen of the organic matter with oxygen. This, 

 however, is to bo understood as included in what 1 have said. Carbon and 

 hydrogen occur in nature, as inorganic matter — so far as concerns plants — only 

 in combination with oxygen as carbonic acid and water. The forces of life and 

 sunlight give rise to organise fuel, by evolving carbon and hydrogen from car- 

 bonic acid and water, and combustion may, tliereforc, witliout objectionable 

 inaccuracy, be described as the descent or return of fuel to an inorganic con- 

 dition — that is, to carbonic acid and water. 



